(1) Field of the Invention
The invention relates to the use of a sub-resolution phase shift mask to improve image resolution in the fabrication of sub-micron integrated circuits and more particularly to a self aligning method to form the mask using vertical anisotropic etching of an opaque material.
(2) Description of Related Prior Art
As optical lithography advances to 0.5 to 0.35 microns and below new technologies are needed to improve the resolution of the imaging lens. Phase-shifting photomasks have been used to improve resolution as well as depth of focus. A transparent coating of thickness t=0.5L/(n-1) (L=wavelength, n=index of refraction) is placed on a mask. The phase of the electric field of the light passing through the coated area is delayed 180.degree. compared to the electric field of light passing through uncoated regions. At the boundary between the coated and uncoated regions the electric fields cancel producing an electric field null as well as an intensity null. This intensity null produces a sharper image as well as improved depth Of focus at the integrated circuit wafer.
Phase-shifting photomasks are described in U.S. Pat. No. 5,045,417 to Okamoto which shows a variety of phase shifter processes. The article "LITHOGRAPHY'S LEADING EDGE, PART 1; PHASE-SHIFT TECHNOLOGY," published in Semiconductor International, February 1992, pages 42-47 describes a number of types of phase shifters but does not describe a process for forming phase shifters.
While improvements have been made in image resolution and depth of field, the linear dimensions used in integrated circuit technology call for even greater improvements in resolution and increased depth of field.